1. Field of the Invention
The present invention relates to a process for removing nitrogenous components from organic waste water such as sewage, rural waste water, miscellaneous domestic waste water, various industrial waste water and piscicultural waste water. More particularly, it is concerned with a denitrifying activated sludge process for converting a variety of nitrogenous compounds in organic waste water into nitrogen gas by rationally combining physiological actions of nitrifying bacteria and denitrifying bacteria.
2. Description of the Related Arts
The processes for removing various nitrogenous compounds from organic waste water by converting the compounds into nitrogen gas through a denitrifying activated sludge process have hitherto been classified by the combination of two steps of nitrifycation step and denitrification step into uniphase, byphase and terphase activated sludge processes, a variety of recycled nitrification/denitrification processes, a variety of intermittent aeration processes, an oxidation ditch process and the like.
In recent years in regard to the above-mentioned processes, a process has been developed in which a microbe-immobilized carrier is introduced in a bioreaction tank for the purpose of reducing biological treatment time and miniaturizing facilities concerned. Such a microbe carrier is required to have a large specific surface area, and to be enhanced in fluidity, bioaffinity and durability each in a reactor. Thus investigations are made on various ceramics, plastics, high molecular gel and the like.
For instance, there are disclosed a conversion process for nitrogen compounds in liquids by the use of a porous carrier of cellulose derivatives in Japanese Patent Application Laid-Open No.256773/1996 (Heisei-8); a microbe carrier which is made of polypropylene (PP) blended with affinitive high molecular compounds in Japanese Patent Application Laid-Open No. 180280/1998 (Heisei-10); a process for removing nitrogenous components in waste water by using a high molecular gel carrier in which nitrifying bacteria, iron oxidation bacteria and denitrifying bacteria were entrappingly immobilized in Japanese Patent Application Laid-Open No. 180291/1998 (Heisei-10); and a process for nitrifying denitrifying organic waste water by the use of a polyvinyl alcohol(PVA)-based hydrous gel simultaneously imparted with nitrification and denitrification performances in Japanese Patent Application Laid-Open No. 314782/1998 (Heisei-10).
However, a denitrifying activated sludge process prolongs an aerobic tank sludge retention time(hereinafter referred to as xe2x80x9cA-SRTxe2x80x9d) required for nitrification longer than the A-SRT required for BOD removal, thus necessitating a denitrification tank, whereby the retention time in biotreatment tanks as a whole is unfavorably prolonged longer than a standard activated sludge process. Further, the mixed liquid suspended solid concentration (hereinafter referred to as xe2x80x9cMLSS concentrationxe2x80x9d) of activated sludge in biotreatment tanks is increased as high as 2500 to 3000 mg/L higher than in a standard activated sludge process. Accordingly, it is necessary to set the water surface loading therein to a value lower than that in a standard activated sludge process.
In view of the foregoing, in the case of carrying out a denitrifying activated sludge process by the use of an existing treatment unit of a standard activated sludge process, it needs only to increase the amount of microbes in a biotreatment tank by adding a microbe carrier in the tank without increasing MLSS concentration. An example of adding a microbe carrier in a denitrifying activated sludge process is intended mainly to promote nitrification, and the addition of a microbe carrier in an aerobic tank can maintain autotrophic nitrifying bacteria at a high concentration and enhance nitrification performance. In order to further increase the amount of microbes in a tank, it becomes necessary to add a microbe carrier in an anaerobic tank. As to only few examples described above which have hitherto been found, the following reasons are taken into consideration:
{circle around (1)} In an anaerobic tank, where aerational agitation is unusable, a mechanical agitation unit is majorly employed. However, the above-mentioned high molecular gel carrier is unendurable to long-term mechanical agitation because of its low physical strength. On the other hand, a carrier composed of ceramics or plastics has high endurance, but low affinity for denitrifying bacteria, thereby the bacteria must be retained in the form of being caught on the uneven portion of the carrier. As a result, there occurs the floatation of the carrier due to nitrogen gas.
{circle around (2)} Nitrogen gas which is formed by the metabolism of microbes in a denitrification step causes floatation of the carrier, since bubbles are attached to the above-mentioned carrier made of PP or porus carrier which has each numerous uneven portions and a large amount of SS retained.
{circle around (3)} Since heterotrophic anaerobic bacteria such as denitrifying bacteria are prone to form granule, the above-mentioned carrier made of ceramics or plastics having low biocompatibility causes the immobilized microbes to peel off, thus instabilizing the treatment performance.
{circle around (4)} A method for entrappingly immobilizing the microbes such as denitrifying bacteria on the above-mentioned high molecular gel is not practical, since a process for producing the carrier, a preservation method and a transport method are made intricate and troublesome.
In such circumstances, an object of the present invention is to develop a nitrifying denitrifying process which is capable of solving the aforesaid problems of the prior arts; efficiently carrying out the nitrifying denitrifying steps, while a microbe carrier can withstand long-term service; steadily achieving high treatment performances; contriving the compactification of the treatment facility; and thus curtailing the treatment time and treatment cost.
Other objects of the present invention will be obvious from the text of this specification hereinafter disclosed.
The present invention has been accomplished on the basis of such findings that by using the same microbe carrier under both aerobic and anaerobic conditions, it is made possible to firmly immobilize the microbes on the surface of a carrier and thus perform efficient nitrification along with denitrification.
That is to say, the present invention provides a process for nitrifying denitrifying organic waste water to nitrify and denitrify the nitrogenous components in the waste water, which comprises making use of a microbe carrier composed of an organic thermoplastic high molecular compound in both a nitrifying step and a denitrifying step, wherein the compound has, in a state of complete swelling in water, a water absorption rate as defined by the formula (1) being in the range of 50 to 3500%, a compressive yield stress as defined by the formula (2) being at least 1.0 MPa and a particle diameter being in the range of 1.0 to 30 mm;
water absorption rate (%)={weight upon complete swelling in water(g)xe2x88x92bone dry weight(g)}xc3x97100/bone dry weight(g)xe2x80x83xe2x80x83(1)
"sgr"=F/Axe2x80x83xe2x80x83(2)
where "sgr" is the compressive yield stress (MPa) of the carrier;
F is the compressive load (N) upon the compressive collapse of the carrier; and
A is the projected area (mm2) of the carrier before compression test.
Moreover, the present invention further provides a process for nitrifying denitrifying organic waste water which comprises continuously or intermittently feeding the waste water to be treated in a treatment tank under the state of a microbe carrier being retained in the treatment tank, while alternately and repeatedly switching treatment conditions between nitrification and denitrification.